Implants often include various electro-magnetic transducers that may function as an actuator, a sensor, and/or a switch. An example of an implant with an electro-magnetic actuator is a middle ear implant which mechanically drives the ossicular chain. Such a middle ear implant that includes a floating mass transducer was developed by Geoffrey Ball et al., and is shown in FIG. 1 (see U.S. Pat. Nos. 5,913,815; 5,897,486; 5,624,376; 5,554,096; 5,456,654; 5,800,336; 5,857,958; and 6,475,134, each of which is incorporated herein by reference).
As shown in FIG. 1, the floating mass transducer 100 includes a housing 101 and at least one coil 102 and 103 coupled to the housing 101. A magnet 104 disposed within the housing 101 is biased by biasing elements 106. The biasing elements 106 are used in defining a resonance frequency, and also reduce friction between the magnet 104 and the interior surface of the housing 101 that may cause distortion. Electrical signals through the at least one coil 102 and 103 cause the magnet 104 to vibrate relative to the housing 101 along an axis 105. The vibration of the magnet 104 causes inertial vibration of the housing 101, which consequently produces vibrations in the inner ear.
Implants may also include an electro-magnetic sensor. Electro-magnetic sensors may be utilized, without limitation, in a microphone, such as a microphone used in converting the mechanical vibrations of an ossicle in the middle ear into an electrical signal.
Another application of an electro-magnetic sensor may be to detect the stapedius reflex. The stapedius reflex is a reflex in the middle ear typically elicited when exceeding the maximum comfortable loudness level. More, particularly, the tympanic muscle and the so-called stapedius muscle are located in the middle ear. The tympanic muscle is linked to the hammer, the stapedius muscle being connected via a tendon to the stirrup. In case of an excessively high sound pressure, which could damage the inner ear, both muscles contract reflexively, so that the mechanical coupling of the eardrum to the inner ear (and thus also the force transmission) is decreased. In this way, it is possible to protect the inner ear from excessively high sound pressures. This tensing of the stapedius muscle triggered as a result of high sound pressures is also referred to as the stapedius reflex. Medically relevant information about the functional capability of the ear may be obtained from the diagnosis of the stapedius reflex. Furthermore, the measurement of the stapedius reflex is useful for setting and/or calibrating so-called cochlear implants, because the sound energy perceived by a patient may be concluded from the measured stapedius reflex.
Instead of an electro-magnetic sensor, other methods for detection of the stapedius reflex typically require a sophisticated surgical technique and special electrodes for recording the myo-electric evoked response, such as a hook electrode patented by Lenarz et al. (see for example, U.S. Pat. No. 6,208,882), or are inconvenient, such as stapedius reflex detection by external tympanometers. FIG. 2 (prior art) depicts an electro-magnetic sensor which in principle could be employed as a stapedius reflex sensor.
Various problems may arise when an electro-magnetic sensor is used to detect the stapedius reflex. One problem is that measuring the stapedius reflex to calibrate a cochlear implant often is performed over an extended period of time of weeks or more. Thus, the sensor and associated wiring requires repetitious installation and removal from the stapedius.
Additionally, upon a wearer of such an auditory (cochlear or middle ear) prosthesis having to undergo Magnetic Resonance Imaging (MRI) examination, interactions between the implanted electro-magnetic transducer and the applied external MRI magnetic field may, at higher field strength (i.e. above about 1 Tesla), produce three potentially harmful effects:
1. The implanted magnet experiences a torque (T=m×B) that may twist the electro-magnetic transducer out of its position, thereby injuring the implant wearer and/or destroying the mechanical fixation, as shown in FIG. 3 (prior art).
2. Due to the external magnetic field, the implanted magnet becomes partly demagnetized and this may lead to damage or at least to a reduced power efficiency of the electro-magnetic transducer after exposure to the MRI field.
3. Magnetic RF pulses (magnetic field B1 in MRI) emitted by the MR unit can induce voltages in the coil(s) of the electro-magnetic transducer and this may destroy the transducer and/or may harm the patient.
Because of these risks it may be generally forbidden to undergo (at least high-field) MRI examination for patients with an implant with electro-magnetic transducer. This may exclude the patient from certain important diagnosis methods.